Method for forming thermal oxide film on semiconductor substrate

ABSTRACT

The present invention is a method for forming a thermal oxide film on a semiconductor substrate, including: a correlation acquisition step of providing a plurality of semiconductor substrates each having a chemical oxide film having a different constitution formed by cleaning, performing a thermal oxidization treatment under identical thermal oxidization treatment conditions to form a thermal oxide film, and determining a correlation between the constitution of the chemical oxide film and a thickness of the thermal oxide film in advance; a cleaning condition determination step of determining the constitution of the chemical oxide film based on the correlation obtained in the correlation acquisition step so that a thickness of a thermal oxide film to be formed on a semiconductor substrate is a predetermined thickness, and determining cleaning conditions for forming a chemical oxide film having the determined constitution of the chemical oxide film; a substrate cleaning step of cleaning the semiconductor substrate under the determined cleaning conditions; and a thermal oxide film formation step of performing a thermal oxidization treatment on the cleaned semiconductor substrate under conditions identical to the thermal oxidization treatment conditions in the correlation acquisition step to form a thermal oxide film on a surface of the semiconductor substrate. Consequently, a thermal oxide film is formed with the target film thickness with excellent reproducibility.

TECHNICAL FIELD

The present invention relates to: a method for forming a thermal oxidefilm on a semiconductor substrate.

BACKGROUND ART

Accompanying layer-increasing and thinning of semiconductor integratedcircuit devices, various films constituting a device are required to beeven thinner. For example, in Patent Document 1, it is stated that whenbonding a silicon wafer, the used silicon wafer needs to have a surfacehaving OH groups, and Patent Document 1 discloses cleaning the siliconwafer by using an ordinary SC1 cleaning solution to form a natural oxidefilm on the surface. Meanwhile, for example, Patent Document 2discloses, as a method for improving gate characteristics of MOStransistors, a method of cleaning a silicon surface immediately beforeforming a gate oxide film and performing hydrogen termination and thenforming a gate insulator film. Thus, in order to form an extremely thinsilicon oxide film that is uniform in the plane or between substratesand with excellent reproducibility, it is now impossible to ignoreeffects of a natural oxide film or a chemical oxide film (an oxide filmformed by a cleaning solution used in a step of cleaning thesemiconductor substrate) formed on a semiconductor substrate beforehand.

CITATION LIST Patent Literature

-   Patent Document 1: JP H09-063910 A-   Patent Document 2: JP 2000-216156 A-   Patent Document 3: JP 2003-115516 A-   Patent Document 4: JP 2002-270596 A

Non Patent Literature

-   Non Patent Document 1: Takahagi, Shinkuu, 33(11), 854 (1990)

SUMMARY OF INVENTION Technical Problem

The present inventors have actually conducted an investigation andresearch, and it has been discovered that when, for example, a method ofcleaning a semiconductor substrate is different, there is a variation inthe thickness of the thermal oxide film after the cleaning. It has alsobe found that this variation in thermal oxide film thickness does notdepend on the thickness of the natural oxide film or the thickness ofthe chemical oxide film before the thermal oxidization. Therefore, ithas been difficult to control a thermal oxidization process since thevariation in the thickness of the thermal oxide film actually formedcannot be known until the semiconductor substrate is actually subjectedto thermal oxidization and the thickness of the thermal oxide film isevaluated.

In Patent Document 3, since OH groups contained in a CVD oxide film(infrared spectroscopy is employed for the evaluation of OH groups in aCVD oxide film) are removed by heating as water, use for calibration andcontrol of moisture meters is proposed. In the case of Patent Document3, a heat treatment is performed at a low temperature such that OHgroups contained in the CVD oxide film are removed as water in advance,and relationships with the growth of a thermal oxide film are notdiscussed. As described, it is known that OH groups are contained in anoxide film or are a source of water in some cases. However, the CVDoxide film is comparatively thick, and OH groups contained in an oxidefilm as thin as a natural oxide film and the subsequent growth of athermal oxide film are not discussed.

Patent Document 4 discloses that it is possible to determine thecomposition strength of a suboxide of each of Si¹⁺, Si²⁺, and Si³⁺immediately above a silicon substrate from an Si2p spectrum measured byX-ray photoelectron spectroscopy (XPS). However, the object is todetermine the interface roughness between silicon and an oxide film, andPatent Document 4 is irrelevant to the technology according to thepresent invention, where the thickness of a thermal oxide film iscontrolled when performing an oxidation heat treatment.

The present invention has been made to solve the above problems, and anobject thereof is to provide a method for forming a thermal oxide filmon a semiconductor substrate by which it is possible to form a thermaloxide film with the thin target thickness and with excellentreproducibility.

Solution to Problem

The present invention has been made to achieve the object, and providesa method for forming a thermal oxide film on a semiconductor substrate,comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different constitution, subjecting theplurality of semiconductor substrates to a thermal oxidization treatmentunder identical thermal oxidization treatment conditions to form athermal oxide film, and determining a correlation between theconstitution of the chemical oxide film and a thickness of the thermaloxide film in advance;

a cleaning condition determination step of determining the constitutionof the chemical oxide film based on the correlation obtained in thecorrelation acquisition step so that a thickness of a thermal oxide filmto be formed on a semiconductor substrate on which a thermal oxide filmis to be formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedconstitution of the chemical oxide film;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate.

According to such a method for forming a thermal oxide film on asemiconductor substrate, a thermal oxide film can be formed with thethin target thickness with excellent reproducibility. As a result,control of a thermal oxidization process is facilitated.

The present invention has been made to achieve the object, and providesa method for forming a thermal oxide film on a semiconductor substrate,comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of OH groups contained inthe chemical oxide film, subjecting the plurality of semiconductorsubstrates to a thermal oxidization treatment under identical thermaloxidization treatment conditions to form a thermal oxide film, anddetermining a correlation between the amount of OH groups in thechemical oxide film and a thickness of the thermal oxide film inadvance;

a cleaning condition determination step of determining the amount of OHgroups in the chemical oxide film based on the correlation obtained inthe correlation acquisition step so that a thickness of a thermal oxidefilm to be formed on a semiconductor substrate on which a thermal oxidefilm is to be formed is a predetermined thickness, and determiningcleaning conditions for forming a chemical oxide film having thedetermined amount of OH groups;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate.

According to such a method for forming a thermal oxide film on asemiconductor substrate, a thin oxide film with a predeterminedthickness can be formed with excellent reproducibility. As a result,control of a thermal oxidization process is facilitated.

In this event, the amount of OH groups is preferably obtained byperforming an ATR-FT-IR measurement of the chemical oxide film by usinga prism for measuring ATR and is preferably calculated from absorbanceof OH groups around 3300 cm⁻¹.

ATR-FT-IR has higher sensitivity to OH groups present in the surfacecompared with common transmission FT-IR, and therefore, the evaluationof the amount of OH groups can be performed with higher accuracy.

The present invention preferably further comprises, after the substratecleaning step and before the thermal oxide film formation step, anOH-group-amount measurement step of measuring an amount of OH groupscontained in a chemical oxide film formed on the semiconductor substrateby the cleaning performed in the substrate cleaning step.

When the amount of OH groups contained in the chemical oxide film ismeasured between the substrate cleaning step and the thermal oxide filmformation step in this manner, the thermal oxide film can be formed witheven more excellent reproducibility.

The present invention has been made to achieve the object, and providesa method for forming a thermal oxide film on a semiconductor substrate,comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having different stoichiometric proportions ofconstituent elements of the chemical oxide film, subjecting theplurality of semiconductor substrates to a thermal oxidization treatmentunder identical thermal oxidization treatment conditions to form athermal oxide film, and determining a correlation between thestoichiometric proportions of the constituent elements of the chemicaloxide film and a thickness of the thermal oxide film in advance;

a cleaning condition determination step of determining thestoichiometric proportions of the constituent elements of the chemicaloxide film based on the correlation obtained in the correlationacquisition step so that a thickness of a thermal oxide film to beformed on a semiconductor substrate on which a thermal oxide film is tobe formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedstoichiometric proportions;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate.

According to such a method for forming a thermal oxide film on asemiconductor substrate, a thermal oxide film can be formed with thethin target thickness with excellent reproducibility. As a result,control of a thermal oxidization process is facilitated.

In this event, regarding the stoichiometric proportions of theconstituent elements of the chemical oxide film, out of the constituentelements of the chemical oxide film, a peak intensity of a bondingenergy in a state where substrate atoms of the semiconductor substrateare not bonded to oxygen atoms and a state where the substrate atoms arebonded to oxygen atoms to form a suboxide and a peak intensity of abonding energy in a state where the substrate atoms are completelybonded to oxygen atoms can be respectively measured using XPS, and thestoichiometric proportions can be defined as proportions of the measuredpeak intensities.

The XPS method is a method by which the information of the outermostsurface layer of the semiconductor substrate can be evaluated simply andwith high precision. In this manner, the thermal oxide film can beformed with the thin target thickness with more excellentreproducibility.

The present invention has been made to achieve the object, and providesa method for forming a thermal oxide film on a semiconductor substrate,comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of hydrogen atomscontained in the chemical oxide film, subjecting the plurality ofsemiconductor substrates to a thermal oxidization treatment underidentical thermal oxidization treatment conditions to form a thermaloxide film, and determining a correlation between the amount of hydrogenatoms in the chemical oxide film and a thickness of the thermal oxidefilm in advance;

a cleaning condition determination step of determining the amount ofhydrogen atoms in the chemical oxide film based on the correlationobtained in the correlation acquisition step so that a thickness of athermal oxide film to be formed on a semiconductor substrate on which athermal oxide film is to be formed is a predetermined thickness, anddetermining cleaning conditions for forming a chemical oxide film havingthe determined amount of hydrogen atoms;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate.

According to such a method for forming a thermal oxide film on asemiconductor substrate, a thermal oxide film can be formed with thethin target thickness with excellent reproducibility. As a result,control of a thermal oxidization process is facilitated.

In this event, the semiconductor substrate can be a silicon wafer andthe thermal oxide film can be a silicon oxide film.

The inventive method for forming a thermal oxide film on a semiconductorsubstrate is particularly suitable for a silicon oxide film formed on asilicon substrate.

In this event, the amount of hydrogen atoms can be obtained byperforming an RBS measurement of the chemical oxide film and can becalculated from a determined proportion of hydrogen atoms in thechemical oxide film.

According to such a measuring method, the amount of hydrogen atoms canbe evaluated with higher accuracy.

In this event, the amount of hydrogen atoms can be obtained byperforming an ATR-FT-IR measurement of the chemical oxide film by usinga prism for measuring ATR and can be calculated from absorbance of SiH₃groups around 2130 cm⁻¹.

ATR-FT-IR has higher sensitivity to hydrogen atoms present in a chemicaloxide film than common transmission FT-IR, and therefore, the amount ofhydrogen atoms can be evaluated with higher accuracy.

In this event, the present invention can further comprise, after thesubstrate cleaning step and before the thermal oxide film formationstep, a hydrogen-atom-amount measurement step of measuring an amount ofhydrogen atoms contained in a chemical oxide film formed on thesemiconductor substrate by the cleaning performed in the substratecleaning step.

When the amount of hydrogen atoms contained in the chemical oxide filmis measured between the substrate cleaning step and the thermal oxidefilm formation step in this manner, the thermal oxide film can be formedwith further excellent reproducibility.

In this event, the predetermined thickness can be 1 to 10 nm.

When the thickness of the thermal oxide film to be formed is in such arange, a thin thermal oxide film having a constant thickness can beformed with more excellent reproducibility.

Advantageous Effects of Invention

As described above, according to the inventive method for forming athermal oxide film on a semiconductor substrate, a thermal oxide filmcan be formed with the thin target thickness with excellentreproducibility. As a result, control of a thermal oxidization processis facilitated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the amount of OHgroups (relative absorbance at 3300 cm⁻¹) and the thickness of a thermaloxide film.

FIG. 2 is a graph showing the relationship between the concentration ofNH₄OH and the amount of OH groups (relative absorbance at 3300 cm⁻¹).

FIG. 3 is a graph showing the relationship between the concentration ofO₃ and the amount of OH groups (relative absorbance at 3300 cm⁻¹).

FIG. 4 is a graph showing the relationship between the concentration ofNH₄OH and the thickness of a thermal oxide film.

FIG. 5 is a graph showing the relationship between the concentration ofO₃ and the thickness of a thermal oxide film.

FIG. 6 is a graph showing the relationship between the proportion of thepeak intensity of Si^(0 to 3+) and the thickness of a thermal oxidefilm.

FIG. 7 is a graph showing the relationship between the proportion of thepeak intensity of Si⁴⁺ and the thickness of a thermal oxide film.

FIG. 8 is a diagram showing an example of X-ray photoelectronspectroscopy (XPS) measurement.

FIG. 9 is a graph showing an example of an XPS spectrum of a samplehaving a silicon oxide film on a silicon substrate.

FIG. 10 is a graph showing the relationship between the concentration ofNH₄OH and the proportion of the peak intensity of Si^(0 to 3+).

FIG. 11 is a graph showing the relationship between the concentration ofNH₄OH and the proportion of the peak intensity of Si⁴⁺.

FIG. 12 is a graph showing the relationship between the concentration ofO₃ and the proportion of the peak intensity of Si^(0 to 3+).

FIG. 13 is a graph showing the relationship between the concentration ofO₃ and the proportion of the peak intensity of Si⁴⁺.

FIG. 14 is a graph showing the relationship between the amount ofhydrogen atoms (proportion of hydrogen atoms in a chemical oxide film)obtained by RBS measurement and the thickness of a thermal oxide film.

FIG. 15 is a graph showing the relationship between the amount ofhydrogen atoms (absorbance at 2130 cm⁻¹) obtained by ATR-FT-IRmeasurement and the thickness of a thermal oxide film.

FIG. 16 is a graph showing the relationship between the concentration ofNH₄OH and the amount of hydrogen atoms (proportion of hydrogen atoms ina chemical oxide film: RBS measurement).

FIG. 17 is a graph showing the relationship between the concentration ofNH₄OH and the amount of hydrogen atoms (absorbance at 2130 cm⁻¹:ATR-FT-IR measurement).

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in detail, but thepresent invention is not limited thereto.

As described above, there has been required a method for forming athermal oxide film on a semiconductor substrate by which it is possibleto form a thermal oxide film with an intended thin thickness withexcellent reproducibility.

The present inventors have earnestly studied the problem and found outthat a thermal oxide film can be formed to have a thin target thicknesswith excellent reproducibility by the following method, and that as aresult, control of a thermal oxidization process is facilitated, themethod being a method for forming a thermal oxide film on asemiconductor substrate, comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different constitution, subjecting theplurality of semiconductor substrates to a thermal oxidization treatmentunder identical thermal oxidization treatment conditions to form athermal oxide film, and determining a correlation between theconstitution of the chemical oxide film and a thickness of the thermaloxide film in advance;

a cleaning condition determination step of determining the constitutionof the chemical oxide film based on the correlation obtained in thecorrelation acquisition step so that a thickness of a thermal oxide filmto be formed on a semiconductor substrate on which a thermal oxide filmis to be formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedconstitution of the chemical oxide film;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step;

and a thermal oxide film formation step of performing a thermaloxidization treatment on the semiconductor substrate cleaned in thesubstrate cleaning step under conditions identical to the thermaloxidization treatment conditions in the correlation acquisition step toform a thermal oxide film on a surface of the semiconductor substrate.Thus, the present inventors have completed the present invention.

The present inventors have earnestly studied the problem and found outthat according to the following method, it is possible to form a thinthermal oxide film with a constant thickness with excellentreproducibility, and that as a result, control of a thermal oxidizationprocess is facilitated, the method being a method for forming a thermaloxide film on a semiconductor substrate, comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of OH groups contained inthe chemical oxide film, subjecting the plurality of semiconductorsubstrates to a thermal oxidization treatment under identical thermaloxidization treatment conditions to form a thermal oxide film, anddetermining a correlation between the amount of OH groups in thechemical oxide film and a thickness of the thermal oxide film inadvance;

a cleaning condition determination step of determining the amount of OHgroups in the chemical oxide film based on the correlation obtained inthe correlation acquisition step so that a thickness of a thermal oxidefilm to be formed on a semiconductor substrate on which a thermal oxidefilm is to be formed is a predetermined thickness, and determiningcleaning conditions for forming a chemical oxide film having thedetermined amount of OH groups;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate. Thus,the present invention has been completed.

The present inventors have earnestly studied the problem and found outthat according to the following method, a thermal oxide film can beformed with the thin target thickness with excellent reproducibility andthat as a result, control of a thermal oxidization process isfacilitated, the method being a method for forming a thermal oxide filmon a semiconductor substrate, comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having different stoichiometric proportions ofconstituent elements of the chemical oxide film, subjecting theplurality of semiconductor substrates to a thermal oxidization treatmentunder identical thermal oxidization treatment conditions to form athermal oxide film, and determining a correlation between thestoichiometric proportions of the constituent elements of the chemicaloxide film and a thickness of the thermal oxide film in advance;

a cleaning condition determination step of determining thestoichiometric proportions of the constituent elements of the chemicaloxide film based on the correlation obtained in the correlationacquisition step so that a thickness of a thermal oxide film to beformed on a semiconductor substrate on which a thermal oxide film is tobe formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedstoichiometric proportions;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate. Thus,the present invention has been completed.

The present inventors have earnestly studied the problem and found outthat according to the following method, a thermal oxide film can beformed with the thin target thickness with excellent reproducibility andthat as a result, control of a thermal oxidization process isfacilitated, the method being a method for forming a thermal oxide filmon a semiconductor substrate, comprising:

a correlation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of hydrogen atomscontained in the chemical oxide film, subjecting the plurality ofsemiconductor substrates to a thermal oxidization treatment underidentical thermal oxidization treatment conditions to form a thermaloxide film, and determining a correlation between the amount of hydrogenatoms in the chemical oxide film and a thickness of the thermal oxidefilm in advance;

a cleaning condition determination step of determining the amount ofhydrogen atoms in the chemical oxide film based on the correlationobtained in the correlation acquisition step so that a thickness of athermal oxide film to be formed on a semiconductor substrate on which athermal oxide film is to be formed is a predetermined thickness, anddetermining cleaning conditions for forming a chemical oxide film havingthe determined amount of hydrogen atoms;

a substrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and

a thermal oxide film formation step of performing a thermal oxidizationtreatment on the semiconductor substrate cleaned in the substratecleaning step under conditions identical to the thermal oxidizationtreatment conditions in the correlation acquisition step to form athermal oxide film on a surface of the semiconductor substrate. Thus,the present invention has been completed.

Hereinafter, a description will be given with reference to the figures.

The present inventors have earnestly investigated the fact thatvariation occurs in the thickness of the formed thermal oxide film whenthe method of cleaning the semiconductor substrate is different, andfound out that the constitution of the chemical oxide film formed bycleaning the semiconductor substrate has a great influence on thermaloxidization treatment. Thus, the present inventors have completed athermal oxidization method that makes it possible to form a thin thermaloxide film having a predetermined thickness with excellentreproducibility by making use of this phenomenon.

For example, when the semiconductor substrate is a silicon wafer, thechemical oxide film is a silicon oxide film, and can be represented bySiO_(x) (0<x≤2). Elements related to the constitution of the chemicaloxide film are, as results of various analyses, silicon, oxygen, andhydrogen. Here, the “x” in the SiO_(x) of the chemical oxide film willbe referred to as oxygen proportion. The oxidization property of thethermal oxide film to be formed is affected by the oxygen proportion (x)of the chemical oxide film and silicon interface, and the formation rateof the thermal oxide film changes. Fluctuation of the oxygen proportionmeans that elements other than oxygen are present at differentproportions.

Hydrogen is present in the form of Si—H or Si—OH. That is, if these Hsincrease, the proportion of oxygen present is affected, and decreases.The proportion of H is small compared with other constituent elements,oxygen and silicon. However, by terminating the silicon in the form ofSi—H or being present as a back bond of silicon, H has an effect on thebonding state of silicon. In addition, H exists as a functional group asin an OH group, and has an important role of determining reactivity.

Meanwhile, silicon and oxygen, which are main constituent elements, havea different bonding ratio to SiO₂, and are called suboxide. Suboxide hasa function as a precursor of a silicon oxide film, and is an importantconstituent that determines the properties of the thermal oxide film tobe formed. In this manner, by focusing on the proportion of oxygen inthe chemical oxide film and obtaining the correlation between theconstitution of the chemical oxide film and the thickness of the thermaloxide film, the thickness of the thermal oxide film can be controlled.

In the present invention, it is sufficient for the semiconductorsubstrates provided for obtaining the correlation to have a chemicaloxide film having a different constitution, and the constitutionincludes the amount of OH groups, the stoichiometric proportions of theconstituent elements, and the amount of hydrogen atoms.

Note that in the present description, the oxide film formed by cleaningthe semiconductor substrate is defined as a chemical oxide film. Here,the method and conditions of the cleaning are not particularly limited.Oxide films formed by cleaning using a chemical solution, cleaning withpure water, etc. are included.

Specific embodiments of the present invention will be described.

First Embodiment

In a method for forming a thermal oxide film on a semiconductorsubstrate according to the first embodiment of the present invention,the constitution of a chemical oxide film formed on a surface of acleaned semiconductor substrate is assessed in advance before thethermal oxidization treatment of the semiconductor substrate. Thecorrelation between the constitution of the chemical oxide film and thethickness of a thermal oxide film when the semiconductor substrate hasbeen subjected to thermal oxidization is determined. Thus, theconstitution of the chemical oxide film on a surface of a semiconductorsubstrate on which a thermal oxide film is to be formed is determined sothat the thickness of the thermal oxide film to be formed on thesemiconductor substrate will be a predetermined thickness. The cleaningconditions are adjusted so as to achieve the determined constitution ofthe chemical oxide film, and a chemical oxide film is formed. In thismanner, it is possible to form a thin thermal oxide film of apredetermined thickness with excellent reproducibility.

The present inventors have earnestly investigated the fact thatvariation occurs in the thickness of the formed thermal oxide film whenthe method of cleaning the semiconductor substrate is different, andfound out that the constitution of the chemical oxide film formed bycleaning the semiconductor substrate has a great influence on thermaloxidization treatment. Thus, the present inventors have completed athermal oxidization method that makes it possible to form a thin thermaloxide film having a predetermined thickness with excellentreproducibility by making use of this phenomenon.

The method for forming a thermal oxide film on a semiconductor substrateaccording to the first embodiment of the present invention will bedescribed.

(Correlation Acquisition Step)

Firstly, a plurality of semiconductor substrates are prepared. Siliconwafers are preferably used as the semiconductor substrates. In thiscase, the thermal oxide film to be formed is a silicon oxide film.Silicon wafers are widely used as semiconductor substrates, andparticularly in a device production process, a thermal oxide film issometimes formed. Therefore, a more accurate evaluation can be performedby forming a thermal oxide film and evaluating the silicon wafer itself.

Firstly, in order to achieve a state where the prepared semiconductorsubstrates do not have an oxide film on the surface, the semiconductorsubstrates are preferably cleaned with HF (hydrofluoric acid). Cleaningis further carried out after removing the oxide film by cleaning withHF. Methods for performing the cleaning after the HF cleaning are notparticularly limited. For example, cleaning using a chemical solutionsuch as SC1 cleaning and O₃ cleaning can be performed, or cleaning suchas pure water rinsing can also be performed. A chemical oxide film isformed on the plurality of prepared semiconductor substrates by thecleaning performed after the HF cleaning. In this event, measures aretaken so that the constitutions of the respective chemical oxide filmsof the plurality of semiconductor substrates are different. When thecleaning is performed by a method using a chemical solution, it ispossible to achieve semiconductor substrates having chemical oxide filmswith different constitutions simply. Therefore, this method ispreferable. Since the cleaning treatment is for obtaining thecorrelation, the cleaning is preferably performed employing as manydifferent kinds of cleaning and/or under as many different cleaningconditions as possible. On obtaining a correlation, it is preferable toperform cleaning employing as many different kinds of cleaning and/orunder as many different cleaning conditions as possible and obtain acorrelation between a plurality of cleaning conditions and constitutionsof a chemical oxide film.

Next, the constitution of the chemical oxide film formed by the cleaningis assessed. In this event, as long as the difference in theconstitutions of the chemical oxide films can be made clear, theassessment is not particularly limited.

Next, the plurality of semiconductor substrates each having a chemicaloxide film with a different constitution are subjected to a thermaloxidization treatment under identical thermal oxidization treatmentconditions to form a thermal oxide film. The formation conditions of thethermal oxide film are not particularly limited, and the formation canbe performed by an ordinary method. Then, the thickness of the formedthermal oxide film is measured. The measurement can be performed, forexample, by spectroscopic ellipsometry or the like.

The correlation between the constitution of the chemical oxide film thusdetermined and the thickness of the formed thermal oxide film isdetermined.

Note that the assessment of the constitution of the chemical oxide filmand the measurement of the thickness of the thermal oxide film can alsobe carried out by using a monitor wafer or the like that has beensubjected to the same cleaning treatment and thermal oxidizationtreatment as the semiconductor substrate on which a thermal oxide filmis to be formed, or by extracting some semiconductor substrates thathave been subjected to the same treatments.

(Cleaning Condition Determination Step)

Cleaning conditions are determined, such that the thickness of thethermal oxide film to be formed on a surface of a semiconductorsubstrate is a predetermined thickness. On the basis of the correlationobtained in the correlation acquisition step, the constitution of thechemical oxide film at which a thermal oxide film having a predeterminedthickness is formed is determined, and in the meantime, cleaningconditions for forming a chemical oxide film so as to achieve thedetermined constitution of the chemical oxide film are determined. Inthe determination of the cleaning conditions, a correlation between thecleaning conditions and the constitution of the chemical oxide film canbe obtained in advance, and this correlation can be used, for example.

(Substrate Cleaning Step)

Next, a semiconductor substrate on which a thermal oxide film is to beactually formed is newly prepared, and cleaning is performed under thecleaning conditions determined in the cleaning condition determinationstep.

(Thermal Oxide Film Formation Step)

A thermal oxidization treatment is performed under the same conditionsas in the thermal oxidization treatment performed in the correlationacquisition step to form a thermal oxide film on a surface of thesemiconductor substrate cleaned in the substrate cleaning step.

By forming a thermal oxide film on the semiconductor substrate throughthe steps according to the present invention as described above, it ispossible to form a thin thermal oxide film of a predetermined thicknesswith excellent reproducibility.

Note that in the present invention, a more remarkable effect can beachieved when the thickness of the thermal oxide film to be formed onthe semiconductor substrate surface is thin and in the range of 1 to 10nm. Therefore, the present invention is suitable for forming a thermaloxide film in such a range.

Second Embodiment

In a method for forming a thermal oxide film on a semiconductorsubstrate according to the second embodiment of the present invention,focusing on the difference in the amount of OH groups contained in thechemical oxide film, the correlation between the amount of OH groups inthe chemical oxide film formed on the surface of the cleanedsemiconductor substrate and the thickness of the thermal oxide film whenthe semiconductor substrate has been subjected to thermal oxidization isdetermined in advance before the thermal oxidization treatment of thesemiconductor substrate. The amount of OH groups in the chemical oxidefilm is determined so that the thickness of the thermal oxide film to beformed on the semiconductor substrate is a predetermined thickness, anda chemical oxide film that has the determined amount of OH groups isformed. In this manner, it is possible to form an oxide film having apredetermined thickness with excellent reproducibility.

The present inventors have earnestly investigated the fact thatvariation occurs in the thickness of the formed thermal oxide film whenthe method of cleaning the semiconductor substrate is different, andfound out that the amount of OH groups in the chemical oxide film formedby cleaning the semiconductor substrate has a great influence on thermaloxidization treatment.

FIG. 1 is a graph showing the relationship between the amount of OHgroups (relative absorbance at 3300 cm⁻¹) in a chemical oxide film on asilicon wafer surface and the thickness of a silicon thermal oxide film.It can be observed that as the relative absorbance at 3300 cm⁻¹increases, the thermal oxide film becomes thicker. This phenomenon issimilar to the oxidization rate being higher in Wet oxidization than inDry oxidization in the case of thermal oxidization using oxidizing gas,and it can be considered that the thickness of the thermal oxide filmafter the thermal oxidization treatment varies depending on thedifference in the amount of OH groups contained in the chemical oxidefilm formed on the silicon wafer surface.

Note that the amount of OH groups contained in the chemical oxide filmcan be determined by, for example, investigating infrared absorptionproperties of the chemical oxide film. As measurement of infraredabsorption properties, for example, FT-IR measurement can be performed,and the amount of OH groups can be calculated from the relativeabsorbance around 3300 cm⁻¹. In this case, the value of the relativeabsorbance around 3300 cm⁻¹ can be used as an index indicating theamount of OH groups. In the following description, “the relativeabsorbance around 3300 cm⁻¹” is also sometimes expressed as “the amountof OH groups”.

The method for forming a thermal oxide film on a semiconductor substrateaccording to the second embodiment of the present invention will bedescribed.

(Correlation Acquisition Step)

Firstly, a plurality of semiconductor substrates are prepared in thesame manner as in the first embodiment. As the semiconductor substrates,silicon wafers are preferably used. In this case, the thermal oxide filmto be formed is a silicon oxide film. Silicon wafers are widely used assemiconductor substrates, and particularly in a device productionprocess, a thermal oxide film is sometimes formed. Therefore, a moreaccurate evaluation can be performed by forming a thermal oxide film andevaluating the silicon wafer itself.

Next, in order to achieve a state where the prepared semiconductorsubstrates do not have an oxide film on the surface, the semiconductorsubstrates are preferably cleaned with HF (hydrofluoric acid). Cleaningis further carried out after removing the oxide film by cleaning withHF. Methods for performing the cleaning after the HF cleaning are notparticularly limited. For example, cleaning using a chemical solutionsuch as SC1 cleaning and O₃ cleaning can be performed, or cleaning suchas pure water rinsing can also be performed. A chemical oxide film isformed on the plurality of prepared semiconductor substrates by thecleaning performed after the HF cleaning. In this event, measures aretaken so that the amount of OH groups contained in the respectivechemical oxide films of the plurality of semiconductor substrates aredifferent. When the cleaning is performed by a method using a chemicalsolution, it is possible to achieve semiconductor substrates havingdifferent amounts of OH groups in the chemical oxide films simply, byusing chemical solutions having different concentrations of OH groups.Therefore, this method is preferable. Furthermore, the SC1 cleaning ispreferable since the higher the concentration of NH₄OH and the higherthe alkalinity, the greater the absorbance at 3300 cm⁻¹ (that is, themore OH groups are contained), and semiconductor substrates havingdifferent amounts of OH groups in the chemical oxide film can beachieved more simply by changing the concentration of NH₄OH. Since therange in which a correlation between the concentration of the chemicalsolution and the amount of OH groups can be obtained varies for eachcleaning method, it is preferable to perform cleaning employing as manydifferent kinds of cleaning and/or under as many different cleaningconditions as possible and obtain a correlation between a plurality ofcleaning conditions and amounts of OH groups when obtaining acorrelation.

Next, the amount of OH groups contained in the respective chemical oxidefilm formed by each cleaning method is measured. In this event, theATR-FT-IR measurement of the chemical oxide film is preferably performedby using a prism for measuring ATR. ATR-FT-IR measurement can performthe evaluation with sufficient sensitivity to the OH groups present inthe semiconductor substrate surface compared with common transmissionFT-IR.

FIG. 2 is a graph showing the relationship between the concentration ofNH₄OH and the amount of OH groups (relative absorbance at 3300 cm⁻¹) ina chemical oxide film, and FIG. 3 is a graph showing the relationshipbetween the concentration of O₃ and the amount of OH groups (relativeabsorbance at 3300 cm⁻¹) in a chemical oxide film. There is acorrelation between the NH₄OH concentration and the amount of OH groups(relative absorbance around 3300 cm⁻¹), but on the other hand, nocorrelation can be observed between the O₃ concentration and the amountof OH groups (relative absorbance around 3300 cm⁻¹). In this manner,there is no correlation between the concentration of the chemicalsolution and the amount of OH groups in some cases depending on thecleaning method.

Next, the plurality of semiconductor substrates each having a differentamount of OH groups contained in the chemical oxide film are subjectedto a thermal oxidization treatment under identical thermal oxidizationtreatment conditions to form a thermal oxide film. The formationconditions of the thermal oxide film are not particularly limited, andthe formation can be performed by an ordinary method. Then, thethickness of the formed thermal oxide film is measured. The measurementcan be performed, for example, by spectroscopic ellipsometry or thelike.

The correlation between the amount of OH groups in the chemical oxidefilm determined above and the thickness of the formed thermal oxide filmis determined. A correlation as in FIG. 1 can be observed between thethickness of the thermal oxide film and the amount of OH groups in thechemical oxide film (relative absorbance around 3300 cm⁻¹), and therecan be observed a tendency that the greater the amount of OH groups inthe chemical oxide film, the thicker the thickness of the thermal oxidefilm. Using this result, the amount of OH groups in the chemical oxidefilm is determined so that the thickness of the thermal oxide film to beformed on the semiconductor substrate is a predetermined thickness, anda chemical oxide film that has the determined amount of OH groups isformed. In this manner, a thermal oxide film having a constant thicknesscan be formed.

Note that the measurement of the amount of OH groups (relativeabsorbance near 3300 cm⁻¹) in the chemical oxide film and themeasurement of the thickness of the thermal oxide film can also becarried out by using a monitor wafer or the like that has been subjectedto the same cleaning treatment and thermal oxidization treatment as thesemiconductor substrate on which a thermal oxide film is to be formed,or by extracting some semiconductor substrates that have been subjectedto the same treatments.

(Cleaning Condition Determination Step)

Cleaning conditions are determined, such that the thickness of thethermal oxide film to be formed on a surface of a semiconductorsubstrate is a predetermined thickness in the same manner as in thefirst embodiment. On the basis of the correlation obtained in thecorrelation acquisition step, the amount of OH groups at which a thermaloxide film having a predetermined thickness is formed is determined, andin the meantime, cleaning conditions for forming a chemical oxide filmso as to achieve the determined amount of OH groups are determined. Inthe determination of the cleaning conditions, a correlation between thecleaning conditions and the amount of OH groups can be obtained inadvance, and this correlation can be used, for example. For example, inthe above-described specific example, when the predetermined thicknessof the thermal oxide film intended to be formed is 5.1 nm, it can beseen from the correlation between the thermal oxide film thickness andthe amount of OH groups obtained in the correlation acquisition step asin FIG. 1 that the amount of OH groups should be 0.145. Accordingly,cleaning conditions where the amount of OH groups is 0.145 can beselected and determined by using the relationship shown in FIG. 2 and soforth. In this specific example, SC1 cleaning where the concentration ofNH₄OH is 0.001 to 0.03% is determined to be performed.

(Substrate Cleaning Step)

Next, in the same manner as in the first embodiment, a semiconductorsubstrate on which a thermal oxide film is to be actually formed isnewly prepared, and cleaning is performed under the cleaning conditionsdetermined in the cleaning condition determination step.

(OH-Group-Amount Measurement Step)

When the amount of OH groups contained in the chemical oxide film formedon the semiconductor substrate by the cleaning performed in thesubstrate cleaning step is measured, the actual amount of OH groups canbe confirmed before performing thermal oxidization. Thus, a thermaloxide film can be formed with further excellent reproducibility. Forexample, when the amount deviates from the target amount of OH groups,it is also possible to perform HF cleaning to remove the oxide filmonce, then, once again, determine the cleaning conditions and clean thesubstrate, and perform the formation of a chemical oxide film that hasan amount closer to the target amount of OH groups. Note that, in thiscase, too, the measurement of the amount of OH groups (relativeabsorbance around 3300 cm⁻¹) in the chemical oxide film can be carriedout by using a monitor wafer or the like that has been subjected to thesame cleaning treatment as the semiconductor substrate on which athermal oxide film is to be formed, or by extracting some semiconductorsubstrates that have been subjected to the same treatments. However,this step does not necessarily need to be performed.

(Thermal Oxide Film Formation Step)

Finally, in the same manner as in the first embodiment, a thermaloxidization treatment is performed under the same conditions as in thethermal oxidization treatment performed in the correlation acquisitionstep to form a thermal oxide film on the surface of the semiconductorsubstrate.

By forming a thermal oxide film on the semiconductor substrate throughthe steps according to the present invention as described above, it ispossible to form a thin thermal oxide film of a predetermined thicknesswith excellent reproducibility.

Note that in the present invention, a more remarkable effect can beachieved when the thickness of the thermal oxide film to be formed onthe semiconductor substrate surface is thin and in the range of 1 to 10nm. Therefore, it is preferable to form a thermal oxide film of such arange.

Third Embodiment

Meanwhile, in a method for forming a thermal oxide film on asemiconductor substrate according to the third embodiment of the presentinvention, focusing on the difference in the stoichiometric proportionsof the constituent elements of the chemical oxide film, the correlationbetween the stoichiometric proportions of the constituent elements ofthe chemical oxide film formed on the surface of the cleanedsemiconductor substrate and the thickness of the thermal oxide film whenthe semiconductor substrate has been subjected to thermal oxidization isdetermined in advance before the thermal oxidization treatment of thesemiconductor substrate. The stoichiometric proportions of theconstituent elements of the chemical oxide film on the surface of thesemiconductor substrate on which a thermal oxide film is to be formed isdetermined so that the thickness of the thermal oxide film to be formedon the semiconductor substrate is a predetermined thickness, cleaningconditions are adjusted so as to achieve the determined stoichiometricproportions, and a chemical oxide film is formed. In this manner, it ispossible to form a thin thermal oxide film of a predetermined thicknesswith excellent reproducibility.

The present inventors have earnestly investigated the fact thatvariation occurs in the thickness of the formed thermal oxide film whenthe method of cleaning the semiconductor substrate is different, andfound out that the stoichiometric proportions of the constituentelements of the chemical oxide film formed by cleaning the semiconductorsubstrate has a great influence on thermal oxidization treatment. Thus,the present inventors have completed a thermal oxidization method thatmakes it possible to form a thin thermal oxide film having apredetermined thickness with excellent reproducibility by making use ofthis phenomenon.

The method for forming a thermal oxide film on a semiconductor substrateaccording to the third embodiment of the present invention will bedescribed.

(Correlation Acquisition Step)

Firstly, a plurality of semiconductor substrates are prepared in thesame manner as in the first embodiment. Silicon substrates arepreferably used as the semiconductor substrates. In this case, thethermal oxide film to be formed is a silicon oxide film. Siliconsubstrates are widely used as semiconductor substrates, and particularlyin a device production process, a thermal oxide film is sometimesformed. Therefore, a more accurate evaluation can be performed byforming a thermal oxide film and evaluating the silicon substrateitself.

Next, in order to achieve a state where the prepared semiconductorsubstrates do not have an oxide film on the surface, the semiconductorsubstrates are preferably cleaned with HF (hydrofluoric acid). Cleaningis further carried out after removing the oxide film by cleaning withHF. Methods for performing the cleaning after the HF cleaning are notparticularly limited. For example, cleaning using a chemical solutionsuch as SC1 cleaning and O₃ cleaning can be performed, or cleaning suchas pure water rinsing can also be performed. A chemical oxide film isformed on the plurality of prepared semiconductor substrates by thecleaning performed after the HF cleaning. In this event, measures aretaken so that the stoichiometric proportions of the constituent elementsof the respective chemical oxide films of the plurality of semiconductorsubstrates are different. When the cleaning is performed by a methodusing a chemical solution, it is possible to achieve semiconductorsubstrates having different stoichiometric proportions of theconstituent elements of the chemical oxide film simply, by using variouskinds of chemical solutions having different concentrations. Therefore,this method is preferable. Since the cleaning treatment is for obtainingthe correlation, the cleaning is preferably performed employing as manydifferent kinds of cleaning and/or under as many different cleaningconditions as possible. In addition, since the range in which acorrelation between the concentration of the chemical solution and thestoichiometric proportions can be obtained varies depending on thecleaning method, it is preferable to perform the cleaning employing asmany different kinds of cleaning and/or under as many different cleaningconditions as possible and obtain the correlation between a plurality ofcleaning conditions and stoichiometric proportions when obtaining thecorrelation.

Next, the stoichiometric proportions of the constituent elements of thechemical oxide film formed by the cleaning is determined.

Note that the methods for determining and evaluating the stoichiometricproportions of the constituent elements of the chemical oxide film arenot particularly limited, and any method is possible as long as thestoichiometric proportions of the constituent elements of the chemicaloxide film can be determined. For example, an XPS method is a method bywhich it is possible to evaluate the information of the outermostsurface layer of the semiconductor substrate simply and with highprecision, and can be employed suitably for the evaluation of thestoichiometric proportions according to the present invention. Out ofthe constituent elements of the chemical oxide film, a peak intensity ofa bonding energy in a state where substrate atoms of the semiconductorsubstrate are not bonded to oxygen atoms and a state where the substrateatoms are bonded to oxygen atoms to form a suboxide and a peak intensityof a bonding energy in a state where the substrate atoms are completelybonded to oxygen atoms can be respectively measured using XPS, and thestoichiometric proportions according to the present invention can bedefined as proportions of the measured peak intensities. Alternatively,it is also possible to, for example, irradiate the surface of thesemiconductor substrate with He ions by RBS, determine the energy of theatoms that collided from the range of the atoms, and determine thestoichiometric proportions of the constituent elements of the chemicaloxide film formed on the semiconductor substrate surface therefrom.

In addition, when the semiconductor substrate is a silicon substrate andthe oxide film to be formed is a silicon oxide film, constituentelements of the chemical oxide film are Si and O. In this event, thestoichiometric proportions can be defined as the proportions ofatom-bonding states of Si atoms and O atoms in the chemical oxide film,that is, the proportion of an Si—Si bond in a state where silicon atomsare not bonded to oxygen atoms and a so-called suboxide out of Si—Obonds (silicon oxide) in a state where silicon atoms are bonded tooxygen atoms and SiO₂ formed by complete bonding with oxygen atoms outof Si—O bonds. The proportion at which the bonds exist can be determinedby measuring the peak intensity of the bonding energy by XPS.

An XPS method is, as shown in an example in FIG. 8 , a technique ofanalyzing the composition of elements constituting a surface of a sampleor the state of chemical bonds by irradiating the surface of the sample(the surface of the silicon oxide film 3 formed on the silicon 4) withX-ray from an X-ray source 1 and detecting photoelectrons (fromoutermost electrons) released from the sample surface with a detector 2,and measuring kinetic energy. The X-ray source used for the irradiationin this event is not particularly limited, and as long as thestoichiometric proportions of the constituent elements of the chemicaloxide film intended to be measured can be measured, an X-ray source ofany energy can be used. Furthermore, the kinetic energy of the releasedphotoelectrons is affected by the state of the electrons around theatoms such as electric charge (valence) of atoms, the distance betweenatoms, etc. By observing the change in energy (chemical shift), thestate of chemical bonds can be discerned relatively easily. The meanfree path of photoelectrons is said to be 2.1 nm in silicon and 3.3 nmin a silicon oxide film, and is considered to be a techniqueparticularly suitable for evaluating the outermost surface of a siliconsubstrate.

FIG. 9 shows an example of an XPS spectrum of a sample in which a thinsilicon oxide film is present on a silicon substrate. FIG. 9 illustratesthe energy range of the sp3 orbital where the outermost electrons of thesilicon are. It is the outermost electrons that contribute to reaction,and inner-shell electrons, which do not contribute to reaction, havebeen omitted. The horizontal axis is the bonding energy and the verticalaxis is the count number of photoelectrons. Since bonding energy variesdepending on the bonding state of Si and O, it is possible to evaluatethe bonding state and bonded atoms. In addition, the vertical axis isthe count number of photoelectrons, and varies depending on the numberof each bonding state.

When the chemical oxide film is a silicon oxide film, it is possible todivide into a bonding state caused by Si—Si bonds of 99 to 100 eV (Si⁰)and a bonding state corresponding to the state in which silicon atoms of101 to 105 eV are bonded to oxygen atoms (Si^(1+ to 4+)) Here, the peakof the Si—Si bonds of Si⁰ is separated in two because of spin-orbitinteraction. In addition, a state in which one oxygen atom is bonded toa silicon atom is Si¹⁺, and a state of SiO₂ in which four oxygen atomsare bonded to a silicon atom is Si⁴⁺. Here, the reason why four bondingstates of silicon atoms and oxygen atoms exist is that the oxide film isthin, and does not necessarily have a stoichiometric composition.

Spin-orbit interaction also occurs in Si—O bonds, but is not observed inordinary XPS due to energy resolution. The bonding energy of Si¹⁺ toSi³⁺, corresponding to suboxide has low intensity, cannot be seenclearly. However, the present energy is known from past knowledge, andspectral separation is performed regarding the intensity of each peak,and the intensity is thus determined.

On determination of the stoichiometric proportions of the constituentelements of the silicon oxide film on the silicon substrate, that is,the proportion of the peak intensity of the bonding energy of Si and O,the peak intensity of Si⁴⁺, being a composition of SiO₂ and the peakintensities of each of Si⁰ to Si³⁺, which can be oxidized by oxygen,were added up. Regarding Si^(1+ to 3+), which do not have clear peaks,spectral separation was performed. That is, Si components that have thepossibility of being oxidized were all added up as Si^(0 to 3+) andseparated from the Si⁴⁺ component, the Si⁴⁺ component having progressedin oxidization and achieved complete stoichiometry. The area of the peakintensity determined as in FIG. 9 was determined and defined as theproportion of the peak intensity.

All of the proportions of the peak intensity of Si^(0 to 3+) obtained inthe above manner and the peak intensity of Si⁴⁺ can be added up, and theproportions (percentages) of each of Si^(0 to 3+) and Si⁴⁺ can bedetermined as the proportion of the peak intensities. The correlationbetween this proportion of peak intensities and the thickness of thethermal oxide film is obtained.

Next, the plurality of semiconductor substrates each having differentstoichiometric proportions of the constituent elements of the chemicaloxide film are subjected to a thermal oxidization treatment under thesame thermal oxidization treatment conditions to form a thermal oxidefilm. The conditions for the formation of the thermal oxide film are notparticularly limited, and the formation can be performed by an ordinarymethod. Then, the thickness of the formed thermal oxide film ismeasured. The measurement can be performed, for example, byspectroscopic ellipsometry or the like.

The correlation between the stoichiometric proportions of theconstituent elements of the chemical oxide film determined above, thatis, the proportions of the peak intensities, and the thickness of theformed thermal oxide film is obtained. FIG. 6 is a graph showing therelationship between the proportion of the peak intensity ofSi^(0 to 3+) and the thickness of the thermal oxide film, and FIG. 7 isa graph showing the relationship between the proportion of the peakintensity of Si⁴⁺ and the thickness of the thermal oxide film. Acorrelation can be seen between the thickness of the thermal oxide filmand the stoichiometric proportions of the constituent elements of thechemical oxide film as in FIG. 6 and FIG. 7 , and it can be observedthat as the proportion of the peak intensity of Si^(0 to 3+) increases,the thickness of the thermal oxide film becomes thicker. In addition, itcan be observed that as the proportion of the peak intensity of Si⁴⁺decreases, the thickness of the thermal oxide film becomes thicker.Using this result, the stoichiometric proportions of the constituentelements of the chemical oxide film are determined so that the thicknessof the thermal oxide film formed on the semiconductor substrate will bea predetermined thickness, and a chemical oxide film that can achievethe determined stoichiometric proportions is formed. Thus, a thinthermal oxide film having a constant thickness can be formed.

Note that the analysis of the stoichiometric proportions of theconstituent elements of the chemical oxide film and the measurement ofthe thickness of the thermal oxide film can also be carried out by usinga monitor wafer or the like that has been subjected to the same cleaningtreatment and thermal oxidization treatment as the semiconductorsubstrate on which a thermal oxide film is to be formed, or byextracting some semiconductor substrates that have been subjected to thesame treatments.

(Cleaning Condition Determination Step)

Cleaning conditions are determined, such that the thickness of thethermal oxide film to be formed on a surface of a semiconductorsubstrate is a predetermined thickness in the same manner as in thefirst embodiment. On the basis of the correlation obtained in thecorrelation acquisition step, the proportions of the peak intensities ofthe bonds of Si^(0 to 3+) and/or Si⁴⁺, that is, the stoichiometricproportions of the constituent elements of the chemical oxide film atwhich a thermal oxide film having a predetermined thickness is formed isdetermined. In the meantime, cleaning conditions for forming a chemicaloxide film so as to achieve the determined proportions of peakintensities are determined. In the determination of the cleaningconditions, a correlation between the cleaning conditions and theproportions of peak intensities can be obtained in advance, and thiscorrelation can be used, for example. For example, in theabove-described specific example, when the target thickness of thethermal oxide film is 5.15 nm, it can be seen from the correlationbetween the thermal oxide film thickness and the proportion of the peakintensity of bonds of Si^(0 to 3+) obtained in the correlationacquisition step as in FIG. 6 that the proportion of the peak intensityof bonds of Si^(0 to 3+) should be 84.5%. Accordingly, cleaningconditions where the proportion of the peak intensity is 84.5% can beselected and determined by using the relationship shown in FIG. 10 andso forth. In this specific example, SC1 cleaning where the concentrationof NH₄OH is 3% is determined to be performed.

(Substrate Cleaning Step)

Next, in the same manner as in the first embodiment, a semiconductorsubstrate on which a thermal oxide film is to be actually formed isnewly prepared, and cleaning is performed under the cleaning conditionsdetermined in the cleaning condition determination step.

(Thermal Oxide Film Formation Step)

Finally, in the same manner as in the first embodiment, a thermaloxidization treatment is performed under the same conditions as in thethermal oxidization treatment performed in the correlation acquisitionstep to form a thermal oxide film on the surface of the semiconductorsubstrate cleaned in the substrate cleaning step.

By forming a thermal oxide film on the semiconductor substrate throughthe steps according to the present invention as described above, it ispossible to form a thin thermal oxide film of a predetermined thicknesswith excellent reproducibility.

Note that in the present invention, a more remarkable effect can beachieved when the thickness of the thermal oxide film to be formed onthe semiconductor substrate surface is thin and in the range of 1 to 10nm. Therefore, the present invention is suitable for forming a thermaloxide film in such a range.

Fourth Embodiment

Meanwhile, in a method for forming a thermal oxide film on asemiconductor substrate according to the fourth embodiment of thepresent invention, focusing on the amount of hydrogen atoms contained inthe chemical oxide film, the amount of hydrogen atoms in the chemicaloxide film formed on the surface of the cleaned semiconductor substrateis measured, and the correlation between the amount of hydrogen atomsand the thickness of the thermal oxide film when the semiconductorsubstrate has been subjected to thermal oxidization is determined inadvance before the thermal oxidization treatment of the semiconductorsubstrate. Thus, the amount of the hydrogen atoms in the chemical oxidefilm on a surface of a semiconductor substrate on which a thermal oxidefilm is to be formed is determined so that the thickness of the thermaloxide film to be formed on the semiconductor substrate will be apredetermined thickness. The cleaning conditions are adjusted so as toachieve the determined amount of hydrogen atoms, and a chemical oxidefilm is formed. In this manner, it is possible to form a thin thermaloxide film of a predetermined thickness with excellent reproducibility.

The present inventors have earnestly investigated the fact thatvariation occurs in the thickness of the formed thermal oxide film whenthe method of cleaning the semiconductor substrate is different, andfound out that the amount of hydrogen atoms in the chemical oxide filmformed by cleaning the semiconductor substrate has a great influence onthermal oxidization treatment. Thus, the present inventors havecompleted a thermal oxidization method that makes it possible to form athin thermal oxide film having a predetermined thickness with excellentreproducibility by making use of this phenomenon.

The method for forming a thermal oxide film on a semiconductor substrateaccording to the fourth embodiment of the present invention will bedescribed.

(Correlation Acquisition Step)

Firstly, a plurality of semiconductor substrates are prepared in thesame manner as in the first embodiment. As the semiconductor substrates,silicon wafers are preferably used. In this case, the thermal oxide filmto be formed is a silicon oxide film. Silicon wafers are widely used assemiconductor substrates, and particularly in a device productionprocess, a thermal oxide film is sometimes formed. Therefore, a moreaccurate evaluation can be performed by forming a thermal oxide film andevaluating the silicon wafer itself.

Next, in order to achieve a state where the prepared semiconductorsubstrates do not have an oxide film on the surface, the semiconductorsubstrates are preferably cleaned with HF (hydrofluoric acid). Cleaningis further carried out after removing the oxide film by cleaning withHF. Methods for performing the cleaning after the HF cleaning are notparticularly limited. For example, cleaning using a chemical solutionsuch as SC1 cleaning and O₃ cleaning can be performed, or cleaning suchas pure water rinsing can also be performed. A chemical oxide film isformed on the plurality of prepared semiconductor substrates by thecleaning performed after the HF cleaning. In this event, measures aretaken so that the amount of hydrogen atoms in the respective chemicaloxide films of the plurality of semiconductor substrates are different.When the cleaning is performed by a method using a chemical solution, itis possible to achieve semiconductor substrates having different amountsof hydrogen atoms in the chemical oxide film simply, by using variouskinds of chemical solutions having different concentrations. Therefore,this method is preferable. Furthermore, the SC1 cleaning is preferablesince the higher the concentration of NH₄OH and the higher thealkalinity, the smaller the proportion of hydrogen atoms or absorbanceat 2130 cm⁻¹ (that is, the smaller the amount of hydrogen atomscontained), and semiconductor substrates having different amounts ofhydrogen atoms can be achieved more simply by changing the concentrationof NH₄OH. Since the cleaning treatment is for obtaining the correlation,the cleaning is preferably performed employing as many different kindsof cleaning and/or under as many different cleaning conditions aspossible. In addition, since the range in which a correlation betweenthe concentration of the chemical solution and the amount of hydrogenatoms can be obtained varies depending on the cleaning method, it ispreferable to perform the cleaning employing as many different kinds ofcleaning and/or under as many different cleaning conditions as possibleand obtain the correlation between a plurality of cleaning conditionsand the amount of hydrogen atoms when obtaining the correlation.

Next, the amount of hydrogen atoms in the chemical oxide film formed bythe cleaning is determined.

Note that the methods for determining and evaluating the amount ofhydrogen atoms in the chemical oxide film are not particularly limited,and any method is possible as long as the amount of hydrogen atoms inthe chemical oxide film can be determined. For example, the amount ofhydrogen atoms can be determined by studying the infrared absorptionproperties of the chemical oxide film. To assess infrared absorptionproperties, for example, ATR-FT-IR measurement can be performed, and theamount of hydrogen atoms can be calculated from the absorbance around2130 cm⁻¹. In this case, the absorbance around 2130 cm⁻¹ is the value ofrelative absorbance corresponding to the stretching vibration of Si—H inSiH₃, and can be an index indicating the amount of hydrogen atoms. Asanother way of determining the amount of hydrogen atoms, for example,Rutherford Backscattering Spectroscopy (RBS) can be performed todetermine the proportion of hydrogen atoms in the chemical oxide film,and the amount of hydrogen atoms can be calculated. In this case, theproportion of hydrogen atoms can be an index showing the amount ofhydrogen atoms. In the following, “the absorbance at 2130 cm⁻¹” and “theproportion of hydrogen atoms” are sometimes referred to as “the amountof hydrogen atoms”.

Next, the plurality of semiconductor substrates each having a differentamount of hydrogen atoms in the chemical oxide film are subjected to athermal oxidization treatment under the same thermal oxidizationtreatment conditions to form a thermal oxide film. The conditions forthe formation of the thermal oxide film are not particularly limited,and the formation can be performed by an ordinary method. Then, thethickness of the formed thermal oxide film is measured. The measurementcan be performed, for example, by spectroscopic ellipsometry or thelike.

The correlation between the amount of hydrogen atoms in the chemicaloxide film determined above and the thickness of the formed thermaloxide film is determined. FIG. 14 is a graph showing the relationshipbetween the amount of hydrogen atoms (the proportion of hydrogen atomsin the chemical oxide film) determined by RBS measurement and thethickness of the thermal oxide film, and FIG. 15 is a graph showing therelationship between the amount of hydrogen atoms (absorbance at 2130cm⁻¹) determined by ATR-FT-IR measurement and the thickness of thethermal oxide film. A correlation as in FIG. 14 and FIG. 15 can beobserved between the thickness of the thermal oxide film and the amountof hydrogen atoms in the chemical oxide film, and there can be observeda tendency that the greater the amount of hydrogen atoms, the thinnerthe thickness of the thermal oxide film. As a factor for the observationof such a tendency, for example, as shown in Non Patent Document 1, itis known that silicon terminated with hydrogen has a stabilized surfaceand becomes inactivated. From this phenomenon, it can be considered thatthe oxidization rate varies depending on the difference in the amount ofhydrogen atoms contained in the chemical oxide film formed on thesurface by cleaning, and that the film thickness after the thermaloxidization varies even when thermal oxidization is performed under thesame conditions. Using the results of FIG. 14 and FIG. 15 , the amountof hydrogen atoms in the chemical oxide film is determined so that thethickness of the thermal oxide film formed on the semiconductorsubstrate will be a predetermined thickness, and a chemical oxide filmthat can achieve the determined amount of hydrogen atoms is formed.Thus, a thin thermal oxide film having a constant thickness can beformed.

Note that the analysis of the amount of hydrogen atoms in the chemicaloxide film and the measurement of the thickness of the thermal oxidefilm can also be carried out by using a monitor wafer or the like thathas been subjected to the same cleaning treatment and thermaloxidization treatment as the semiconductor substrate on which a thermaloxide film is to be formed, or by extracting some semiconductorsubstrates that have been subjected to the same treatments.

(Cleaning Condition Determination Step)

Cleaning conditions are determined, such that the thickness of thethermal oxide film to be formed on a surface of a semiconductorsubstrate is a predetermined thickness in the same manner as in thefirst embodiment. On the basis of the correlation obtained in thecorrelation acquisition step, the amount of hydrogen atoms at which athermal oxide film having a predetermined thickness is formed isdetermined, and in the meantime, cleaning conditions for forming achemical oxide film so as to achieve the determined amount of hydrogenatoms are determined. In the determination of the cleaning conditions, acorrelation between the cleaning conditions and the amount of hydrogenatoms can be obtained in advance, and this correlation can be used, forexample.

For example, in the above-described specific example, when the targetthickness of the thermal oxide film is 5.10 nm, it can be seen from thecorrelation between the thermal oxide film thickness and the proportionof hydrogen atoms obtained in the correlation acquisition step as inFIG. 14 that the proportion of hydrogen atoms should be 20%.Accordingly, cleaning conditions where the amount of hydrogen atoms is20% can be selected and determined by using the relationship shown inFIG. 16 and so forth. In this specific example, SC1 cleaning where theconcentration of NH₄OH is 0.03% is determined to be performed.

(Substrate Cleaning Step)

Next, in the same manner as in the first embodiment, a semiconductorsubstrate on which a thermal oxide film is to be actually formed isnewly prepared, and cleaning is performed under the cleaning conditionsdetermined in the cleaning condition determination step.

(Hydrogen-Atom-Amount Measurement Step)

When the amount of hydrogen atoms contained in the chemical oxide filmformed on the semiconductor substrate by the cleaning performed in thesubstrate cleaning step is measured, the actual amount of hydrogen atomscan be confirmed before performing thermal oxidization. Thus, a thermaloxide film can be formed with further excellent reproducibility. Forexample, when the amount deviates from the target amount of hydrogenatoms, it is also possible to perform HF cleaning to remove the chemicaloxide film once, then, once again, determine the cleaning conditions andclean the substrate, and perform the formation of a chemical oxide filmthat has an amount closer to the target amount of hydrogen atoms. Notethat, in this case, too, the measurement of the amount of hydrogen atoms(the absorbance around 2130 cm⁻¹ according to an ATR-FT-IR measurementor the proportion of hydrogen atoms according to an RBS measurement) inthe chemical oxide film can be carried out by using a monitor wafer orthe like that has been subjected to the same cleaning treatment as thesemiconductor substrate on which a thermal oxide film is to be formed,or by extracting some semiconductor substrates that have been subjectedto the same treatments. However, this step does not necessarily need tobe performed.

(Thermal Oxide Film Formation Step)

Finally, in the same manner as in the first embodiment, a thermaloxidization treatment is performed under the same conditions as in thethermal oxidization treatment performed in the correlation acquisitionstep to form a thermal oxide film on the surface of the semiconductorsubstrate cleaned in the substrate cleaning step.

By forming a thermal oxide film on the semiconductor substrate throughthe steps according to the present invention as described above, it ispossible to form a thin thermal oxide film of a predetermined thicknesswith excellent reproducibility.

Note that in the present invention, a more remarkable effect can beachieved when the thickness of the thermal oxide film to be formed onthe semiconductor substrate surface is thin and in the range of 1 to 10nm. Therefore, the present invention is suitable for forming a thermaloxide film in such a range.

EXAMPLE

Hereinafter, the present invention will be described specifically withreference to Examples, but the present invention is not limited thereto.

Example 1

A plurality of boron-doped silicon single crystal substrates having adiameter of 300 mm and a usual resistivity were prepared, and aftercleaning the surface of the silicon single crystal substrates with 0.5%HF for initialization, SC1 cleaning was performed at 70° C. In thisevent, the concentration of NH₄OH was altered to be 3, 0.3, 0.03, and0.001%. In addition, as a different cleaning, O₃ cleaning (24° C.) wasperformed with the concentration of O₃ varied to 3, 20, and 40 ppm.

Subsequently, a test piece having a size of a few centimeters square wascut out of each silicon single crystal substrate cleaned under therespective cleaning conditions beforehand and ATR-FT-IR measurement(attenuated total reflectance Fourier transform infrared spectroscopy)was performed to measure the relative absorbance at 3300 cm⁻¹, and theconcentration of NH₄OH and the concentration of O₃ were compared withthe amount of OH groups (relative absorbance around 3300 cm⁻¹) in thechemical oxide film. The results are shown in FIG. 2 and FIG. 3 . Asshown in FIG. 2 , it can be observed that as the NH₄OH concentrationincreases, the amount of OH groups (relative absorbance around 3300cm⁻¹) also increases, and many OH groups are contained. On the otherhand, as shown in FIG. 3 , in the case of O₃, no dependence on theamount of OH groups (relative absorbance around 3300 cm⁻¹) was found inthe O₃ concentration.

The reason why the amount of OH groups (relative absorbance around 3300cm⁻¹) varies depending on the cleaning conditions can be considered tobe that in the case of SC1 cleaning, the higher the NH₄OH concentrationand the higher the alkalinity, the more OH groups are contained, but inthe case of O₃ cleaning, the chemical solution is almost neutral, andthe amount of OH groups is small.

Next, a different wafer that has been cleaned under the same cleaningconditions as the wafers from which a test piece was cut out forATR-FT-IR evaluation was subjected to oxidization (900° C., oxygen: 5%,60 min) with the aim of obtaining a thermal oxide film having athickness of 5.1 nm. Then, the thickness of the thermal oxide film wasmeasured by spectroscopic ellipsometry. The results are shown in FIG. 4and FIG. 5 . FIG. 4 is a graph showing the relationship between theNH₄OH concentration and the thermal oxide film thickness. FIG. 5 is agraph showing the relationship between the O₃ concentration and thethermal oxide film thickness.

From the above results, a correlation between the thickness of thethermal oxide film and the relative absorbance around 3300 cm⁻¹ as shownin FIG. 1 was obtained. As shown in FIG. 1 , a correlation was observedbetween the thermal oxide film thickness and the relative absorbancearound 3300 cm⁻¹, and a tendency was observed that the greater theamount of OH groups in the chemical oxide film formed after thecleaning, the thicker the thermal oxide film. It was revealed that bymaking use of the correlation between the thickness of the thermal oxidefilm and the amount of OH groups (relative absorbance around 3300 cm⁻¹)shown in FIG. 1 to determine the cleaning conditions for actuallyforming a thermal oxide film, it is possible to form a thermal oxidefilm having a thickness close to the target thermal oxide filmthickness.

Next, cleaning conditions of SC1 cleaning under which the thickness ofthe thermal oxide film becomes 5.1 nm were considered. It was shown fromFIG. 1 that to realize this thermal oxide film thickness, the absorbanceat 3300 cm⁻¹ needed to be 0.145, and to achieve this absorbance, it wasshown from FIG. 2 that the concentration of NH₄OH can be 0.001 to 0.03%.Accordingly, when the silicon single crystal substrate was cleaned withthe NH₄OH in the SC1 cleaning adjusted to be 0.03%, and then theabove-described thermal oxidization treatment was performed, thethickness of the thermal oxide film was 5.1 nm, and the same thicknessas the target thickness was successfully achieved.

As shown above, it is possible to form a thermal oxide film having aneven and constant thickness with excellent reproducibility by making useof the correlation determined beforehand to set the thermal oxidizationtreatment conditions to the same conditions as in the correlationacquisition step and adjust the cleaning conditions so that the amountof OH groups at which the target thermal oxide film thickness can beobtained is achieved. As a result, it has been shown that control of athermal oxidization process is facilitated.

Example 2

Boron-doped silicon single crystal substrates having a diameter of 300mm and a usual resistivity were prepared, and after cleaning the surfaceof the silicon substrates with 0.5% HF for initialization, SC1 cleaningwas performed at 70° C. In this event, the concentration of NH₄OH wasaltered to be 3, 0.3, 0.03, and 0.01%. In addition, as a differentcleaning, O₃ cleaning (24° C.) was performed with the concentration ofO₃ varied to 3, 20, and 40 ppm.

Subsequently, a test piece was cut out of each silicon single crystalsubstrate beforehand and XPS measurement was performed to measure thepeak intensities of Si^(0 to 3+) and Si⁴⁺, and the concentration ofNH₄OH and the proportions of the peak intensities of Si^(0 to 3+) and Siwere compared. The results are shown in FIG. 10 and FIG. 11 . FIG. 10 isa graph showing the relationship between the NH₄OH concentration and theproportion of the peak intensity of Si^(0 to 3+) FIG. 11 is a graphshowing the relationship between the NH₄OH concentration and theproportion of the peak intensity of Si⁴⁺. In addition, the peakintensities were measured in the same manner and the concentration of O₃was compared with the proportions of the peak intensities ofSi^(0 to 3+) and Si⁴⁺. The results are shown in FIG. 12 and FIG. 13 .FIG. 12 is a graph showing the relationship between the O₃ concentrationand the proportion of the peak intensity of Si^(0 to 3+). FIG. 13 is agraph showing the relationship between the O₃ concentration and theproportion of the peak intensity of Si⁴⁺. As a result, there is atendency that as the NH₄OH concentration increases, the proportion ofthe peak intensity of Si^(0 to 3+) increases, but on the contrary, theproportion of the peak intensity of Si⁴⁺ decreases. On the other hand,in the case of O₃, no dependence on the proportion of the peak intensitywas found in the O₃ concentration. In this manner, there is nocorrelation between the concentration of the chemical solution and theproportions of the peak intensities of the constituent elements of thechemical oxide film in some cases depending on the cleaning method.

Such substrates were all subjected to thermal oxidization (900° C.,oxygen: 5%, 60 min) with the aim of making the thickness of the thermaloxide film about 5.1 nm, and then the thickness of the thermal oxidefilm was measured by spectroscopic ellipsometry.

Correlations as shown in FIG. 6 and FIG. 7 were obtained from theresults of the experiment conducted as described above. As shown in FIG.6 and FIG. 7 , correlations were seen between the thickness of thethermal oxide film and the proportions of the peak intensities ofSi^(0 to 3+) and Si⁴⁺, and it was shown that the greater the proportionof the peak intensity of Si^(0 to 3+), the thicker the thickness ofthermal oxide film tended to be, and that the smaller the proportion ofthe peak intensity of Si⁴⁺, the thicker the thickness of the thermaloxide film tended to be. No dependence of the O₃ concentration on thestoichiometric proportions was observed, but a good correlation was seenbetween the stoichiometric proportions and the thickness of the thermaloxide film formed after O₃ cleaning. In addition, it was found that fromthe correlation between the thickness of the thermal oxide film and theproportion of the peak intensity of the constituent elements of thechemical oxide film shown in FIG. 6 and FIG. 7 , a calibration line suchas the dotted line in FIG. 6 and FIG. 7 can be drawn, for example. Whenthe conditions for actually forming a thermal oxide film are determinedfrom this calibration line, a thermal oxide film close to the targetthermal oxide film thickness can be formed. The dotted lines in FIG. 6and FIG. 7 are calibration lines. The equation for each calibration lineis as follows.

(oxide film thickness nm)=0.0342×(proportion of peak intensity ofSi^(0 to 3+))+2.26

(oxide film thickness nm)=−0.0342×(proportion of peak intensity ofSi⁴⁺)+5.68

Next, cleaning conditions of SC1 cleaning under which the thickness ofthe thermal oxide film becomes 5.15 nm were considered. To realize thisthermal oxide film thickness, the proportion of the peak intensity ofSi^(0 to 3+) needs to be 84.5% according to FIG. 6 , and the proportionof the peak intensity of Si⁴⁺ needs to be 15.5% according to FIG. 7 . Toachieve this value, it was shown from the graph of the NH₄OHconcentration and the proportion of the peak intensity of Si^(0 to 3+)shown in FIG. 10 and the graph of the NH₄OH concentration and theproportion of the peak intensity of Si⁴⁺ shown in FIG. 11 that the NH₄OHconcentration can be 3%. Accordingly, when the silicon single crystalsubstrate was cleaned with the NH₄OH concentration in the SC1 cleaningadjusted to be 3%, and then a thermal oxidization treatment wasperformed under conditions identical to when the correlation wasobtained, the thickness of the thermal oxide film was 5.16 nm, and thetarget thickness was successfully achieved.

As shown above, it is possible to form a thin thermal oxide film havingan even and constant thickness with excellent reproducibility by makinguse of the correlation determined beforehand to set the thermaloxidization treatment conditions to the same conditions as in thecorrelation acquisition step and adjust the cleaning conditions so thatthe stoichiometric proportions of the constituent elements of thechemical oxide film at which the target thermal oxide film thickness canbe obtained is achieved. As a result, it has been shown that control ofa thermal oxidization process is facilitated.

Experimental Example 1

A method for adjusting the thermal oxidization treatment conditions byRBS measurement so that the thickness of the thermal oxide film becomesa target thickness will be described.

Firstly, boron-doped silicon wafers having a diameter of 300 mm and ausual resistivity were prepared, and after cleaning the surface of thesilicon wafers with 0.5% HF for initialization, SC1 cleaning wasperformed at 70° C. In this event, the concentration of NH₄OH wasaltered to be 3, 0.3, 0.03, and 0.001%.

Subsequently, a test piece was cut out of each silicon wafer beforehandand RBS measurement was performed to measure the proportion of hydrogenatoms, and the concentration of NH₄OH was compared with the proportionof hydrogen atoms. The results are shown in FIG. 16 . FIG. 16 is a graphshowing the relationship between the NH₄OH concentration and theproportion of hydrogen atoms in the chemical oxide film determined bythe RBS measurement. As a result, it was observed that as the NH₄OHconcentration increases, the proportion of hydrogen atoms decreases, asshown in FIG. 16 . It can be considered that the proportion of hydrogenatoms varies depending on the cleaning conditions because, in the caseof SC1 cleaning, the higher the NH₄OH concentration and the higher thealkalinity, the fewer the hydrogen atoms.

Such wafers were all subjected to thermal oxidization (900° C., oxygen:5%, 60 min) with the aim of making the thickness of the thermal oxidefilm 5.10 nm, and then the thickness of the thermal oxide film wasmeasured by spectroscopic ellipsometry.

The correlation shown in FIG. 14 was obtained from the results of theexperiment conducted as described above. As shown in FIG. 14 , acorrelation was seen between the thickness of the thermal oxide film andthe proportion of hydrogen atoms in the cleaned chemical oxide film, anda tendency was observed that the larger the proportion of hydrogen atomsin the cleaned chemical oxide film, the thinner the film thickness. Itwas revealed that by making use of the correlation between the thicknessof the thermal oxide film and the amount of hydrogen atoms (theproportion of hydrogen atoms in the chemical oxide film determined byRBS measurement) shown in FIG. 14 to determine the cleaning conditionsfor actually forming a thermal oxide film, it is possible to form athermal oxide film having a thickness close to the target thermal oxidefilm thickness.

Experimental Example 2

Furthermore, as a different method, a method for adjusting the thermaloxidization treatment conditions by ATR-FT-IR measurement so that thethickness of the thermal oxide film becomes a target thickness will bedescribed.

Firstly, silicon wafers identical to the silicon wafers prepared inExperimental Example 1 were prepared. Then, a test piece was cut out ofeach silicon wafer, ATR-FT-IR measurement was performed to measure theabsorbance around 2130 cm⁻¹, and the concentration of NH₄OH was comparedwith the absorbance around 2130 cm⁻¹. The results are shown in FIG. 17 .FIG. 17 is a graph showing the relationship between the NH₄OHconcentration and the absorbance at 2130 cm⁻¹ determined by theATR-FT-IR measurement. As a result, it was observed that as the NH₄OHconcentration increases, the absorbance around 2130 cm⁻¹ decreases, andthe amount of hydrogen atoms in the cleaned chemical oxide filmdecreases.

When such wafers were all subjected to a thermal oxidization treatmentin the same manner as in Experimental Example 1, the correlation asshown in FIG. 15 was obtained. As shown in FIG. 15 , a correlation wasseen between the thickness of the thermal oxide film and the absorbanceof the cleaned chemical oxide film around 2130 cm⁻¹, and a tendency thatthe greater the absorbance of the cleaned chemical oxide film around2130 cm⁻¹, the thinner the film thickness was observed. It was revealedthat by making use of the correlation between the thickness of thethermal oxide film and the amount of hydrogen atoms (absorbance around2130 cm⁻¹) shown in FIG. 15 to determine the cleaning conditions foractually forming a thermal oxide film, it is possible to form a thermaloxide film having a thickness close to the target thermal oxide filmthickness.

Example 3

In the same manner as in the Experimental Example 1, the correlationbetween the proportion of hydrogen atoms measured by RBS and thethickness of the thermal oxide film was determined. Firstly, a pluralityof boron-doped silicon wafers having a diameter of 300 mm and a usualresistivity were prepared, and after cleaning the silicon wafer surfacewith 0.5% HF for initialization, each silicon wafer was subjected to SC1cleaning (70° C., NH₄OH concentration: 3, 0.3, 0.03, and 0.001%). Thus,substrates having different proportions of hydrogen atoms werefabricated. Next, a test piece was cut out from each silicon wafer andsubjected to RBS measurement to measure the proportion of hydrogenatoms. After that, a predetermined thermal oxidization treatment (900°C., oxygen: 5%, 60 min) was performed on each substrate of the differentcleaning conditions, the thickness of each thermal oxide film wasmeasured, and the correlation between the proportion of hydrogen atomsand the thickness of the thermal oxide film was determined. In thisevent, the thickness of the thermal oxide film at which to obtain thecorrelation was set to be around 5.10 nm. In this manner, thecorrelation shown in FIG. 14 was obtained.

From the data obtained in the process of obtaining the correlation, theNH₄OH concentration and the proportion of hydrogen atoms were compared.The results are shown in FIG. 16 . FIG. 16 is a graph showing therelationship between the NH₄OH concentration and the proportion ofhydrogen atoms. As a result, it has been shown that there is a tendencythat as the concentration of NH₄OH increases, the proportion of hydrogenatoms decreases.

Next, cleaning conditions of SC1 cleaning under which the thickness ofthe thermal oxide film becomes 5.10 nm were considered. According toFIG. 14 , the proportion of hydrogen atoms needs to be 20% to realizethis thermal oxide film thickness. To achieve this value, it was shownfrom the graph of the relationship between the NH₄OH concentration andthe proportion of hydrogen atoms shown in FIG. 16 that the NH₄OHconcentration can be 0.03%. Accordingly, when the silicon substrate wascleaned with the NH₄OH concentration in the SC1 cleaning adjusted to be0.03%, and then a thermal oxidization treatment was performed underconditions identical to when the correlation was obtained, the thicknessof the thermal oxide film was 5.10 nm, and the target thickness wassuccessfully achieved.

Example 4

In the same manner as in the Experimental Example 2, the correlationbetween the absorbance around 2130 cm⁻¹ measured by ATR-FT-IR and thethickness of the thermal oxide film was determined. Firstly, a pluralityof boron-doped silicon wafers having a diameter of 300 mm and a usualresistivity were prepared, and after cleaning the silicon wafer surfacewith 0.5% HF for initialization, each silicon wafer was subjected to SC1cleaning (70° C., NH₄OH concentration: 3, 0.3, 0.03, and 0.001%). Thus,substrates having a different absorbance around 2130 cm⁻¹ werefabricated. Next, a test piece was cut out from each silicon wafer andsubjected to ATR-FT-IR measurement to measure the absorbance near 2130cm⁻¹. After that, a predetermined thermal oxidization treatment (900°C., oxygen: 5%, 60 min) was performed on each substrate of the differentcleaning conditions, the thickness of each thermal oxide film wasmeasured, and the correlation between the absorbance around 2130 cm⁻¹and the thickness of the thermal oxide film was determined. In thisevent, the thickness of the thermal oxide film at which to obtain thecorrelation was set to be around 5.10 nm. In this manner, thecorrelation shown in FIG. 15 was obtained.

From the data obtained in the process of obtaining the correlation, theNH₄OH concentration and the absorbance around 2130 cm⁻¹ were compared.The results are shown in FIG. 17 . FIG. 17 is a graph showing therelationship between the NH₄OH concentration and the absorbance around2130 cm⁻¹. As a result, it has been shown that there is a tendency thatas the concentration of NH₄OH increases, the absorbance around 2130 cm⁻¹decreases.

Next, cleaning conditions of SC1 cleaning under which the thickness ofthe thermal oxide film becomes 5.10 nm were considered. According toFIG. 15 , the absorbance around 2130 cm⁻¹ needs to be 1.0 to realizethis thermal oxide film thickness. To achieve this value, it was shownfrom the graph of the relationship between the NH₄OH concentration andthe absorbance around 2130 cm⁻¹ shown in FIG. 17 that the NH₄OHconcentration can be 0.03%. Accordingly, when the silicon substrate wascleaned with the NH₄OH concentration in the SC1 cleaning adjusted to be0.03%, and then a thermal oxidization treatment was performed underconditions identical to when the correlation was obtained, the thicknessof the thermal oxide film was 5.10 nm, and the target thickness wassuccessfully achieved.

As shown in Examples 3 and 4, it is possible to form a thin thermaloxide film having an even and constant thickness with excellentreproducibility by making use of the correlation determined beforehandto set the thermal oxidization treatment conditions to the sameconditions as in the correlation acquisition step and adjust thecleaning conditions so that the amount of hydrogen atoms at which thetarget thermal oxide film thickness can be obtained is achieved. As aresult, it has been shown that control of a thermal oxidization processis facilitated.

It should be noted that the present invention is not limited to theabove-described embodiments. The embodiments are just examples, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept disclosedin claims of the present invention are included in the technical scopeof the present invention.

1-13. (canceled)
 14. A method for forming a thermal oxide film on asemiconductor substrate, comprising: a correlation acquisition step ofproviding a plurality of semiconductor substrates each having a chemicaloxide film formed by cleaning, each chemical oxide film having adifferent constitution, subjecting the plurality of semiconductorsubstrates to a thermal oxidization treatment under identical thermaloxidization treatment conditions to form a thermal oxide film, anddetermining a correlation between the constitution of the chemical oxidefilm and a thickness of the thermal oxide film in advance; a cleaningcondition determination step of determining the constitution of thechemical oxide film based on the correlation obtained in the correlationacquisition step so that a thickness of a thermal oxide film to beformed on a semiconductor substrate on which a thermal oxide film is tobe formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedconstitution of the chemical oxide film; a substrate cleaning step ofcleaning the semiconductor substrate under the cleaning conditionsdetermined in the cleaning condition determination step; and a thermaloxide film formation step of performing a thermal oxidization treatmenton the semiconductor substrate cleaned in the substrate cleaning stepunder conditions identical to the thermal oxidization treatmentconditions in the correlation acquisition step to form a thermal oxidefilm on a surface of the semiconductor substrate.
 15. A method forforming a thermal oxide film on a semiconductor substrate, comprising: acorrelation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of OH groups contained inthe chemical oxide film, subjecting the plurality of semiconductorsubstrates to a thermal oxidization treatment under identical thermaloxidization treatment conditions to form a thermal oxide film, anddetermining a correlation between the amount of OH groups in thechemical oxide film and a thickness of the thermal oxide film inadvance; a cleaning condition determination step of determining theamount of OH groups in the chemical oxide film based on the correlationobtained in the correlation acquisition step so that a thickness of athermal oxide film to be formed on a semiconductor substrate on which athermal oxide film is to be formed is a predetermined thickness, anddetermining cleaning conditions for forming a chemical oxide film havingthe determined amount of OH groups; a substrate cleaning step ofcleaning the semiconductor substrate under the cleaning conditionsdetermined in the cleaning condition determination step; and a thermaloxide film formation step of performing a thermal oxidization treatmenton the semiconductor substrate cleaned in the substrate cleaning stepunder conditions identical to the thermal oxidization treatmentconditions in the correlation acquisition step to form a thermal oxidefilm on a surface of the semiconductor substrate.
 16. The method forforming a thermal oxide film on a semiconductor substrate according toclaim 15, wherein the amount of OH groups is obtained by performing anATR-FT-IR measurement of the chemical oxide film by using a prism formeasuring ATR and is calculated from absorbance of OH groups around 3300cm⁻¹.
 17. The method for forming a thermal oxide film on a semiconductorsubstrate according to claim 15, further comprising, after the substratecleaning step and before the thermal oxide film formation step, anOH-group-amount measurement step of measuring an amount of OH groupscontained in a chemical oxide film formed on the semiconductor substrateby the cleaning performed in the substrate cleaning step.
 18. A methodfor forming a thermal oxide film on a semiconductor substrate,comprising: a correlation acquisition step of providing a plurality ofsemiconductor substrates each having a chemical oxide film formed bycleaning, each chemical oxide film having different stoichiometricproportions of constituent elements of the chemical oxide film,subjecting the plurality of semiconductor substrates to a thermaloxidization treatment under identical thermal oxidization treatmentconditions to form a thermal oxide film, and determining a correlationbetween the stoichiometric proportions of the constituent elements ofthe chemical oxide film and a thickness of the thermal oxide film inadvance; a cleaning condition determination step of determining thestoichiometric proportions of the constituent elements of the chemicaloxide film based on the correlation obtained in the correlationacquisition step so that a thickness of a thermal oxide film to beformed on a semiconductor substrate on which a thermal oxide film is tobe formed is a predetermined thickness, and determining cleaningconditions for forming a chemical oxide film having the determinedstoichiometric proportions; a substrate cleaning step of cleaning thesemiconductor substrate under the cleaning conditions determined in thecleaning condition determination step; and a thermal oxide filmformation step of performing a thermal oxidization treatment on thesemiconductor substrate cleaned in the substrate cleaning step underconditions identical to the thermal oxidization treatment conditions inthe correlation acquisition step to form a thermal oxide film on asurface of the semiconductor substrate.
 19. The method for forming athermal oxide film on a semiconductor substrate according to claim 18,wherein regarding the stoichiometric proportions of the constituentelements of the chemical oxide film, out of the constituent elements ofthe chemical oxide film, a peak intensity of a bonding energy in a statewhere substrate atoms of the semiconductor substrate are not bonded tooxygen atoms and a state where the substrate atoms are bonded to oxygenatoms to form a suboxide and a peak intensity of a bonding energy in astate where the substrate atoms are completely bonded to oxygen atomsare respectively measured using XPS, and the stoichiometric proportionsare defined as proportions of the measured peak intensities.
 20. Themethod for forming a thermal oxide film on a semiconductor substrateaccording to claim 14, wherein the semiconductor substrate is a siliconwafer and the thermal oxide film is a silicon oxide film.
 21. The methodfor forming a thermal oxide film on a semiconductor substrate accordingto claim 15, wherein the semiconductor substrate is a silicon wafer andthe thermal oxide film is a silicon oxide film.
 22. The method forforming a thermal oxide film on a semiconductor substrate according toclaim 18, wherein the semiconductor substrate is a silicon wafer and thethermal oxide film is a silicon oxide film.
 23. A method for forming athermal oxide film on a semiconductor substrate, comprising: acorrelation acquisition step of providing a plurality of semiconductorsubstrates each having a chemical oxide film formed by cleaning, eachchemical oxide film having a different amount of hydrogen atomscontained in the chemical oxide film, subjecting the plurality ofsemiconductor substrates to a thermal oxidization treatment underidentical thermal oxidization treatment conditions to form a thermaloxide film, and determining a correlation between the amount of hydrogenatoms in the chemical oxide film and a thickness of the thermal oxidefilm in advance; a cleaning condition determination step of determiningthe amount of hydrogen atoms in the chemical oxide film based on thecorrelation obtained in the correlation acquisition step so that athickness of a thermal oxide film to be formed on a semiconductorsubstrate on which a thermal oxide film is to be formed is apredetermined thickness, and determining cleaning conditions for forminga chemical oxide film having the determined amount of hydrogen atoms; asubstrate cleaning step of cleaning the semiconductor substrate underthe cleaning conditions determined in the cleaning conditiondetermination step; and a thermal oxide film formation step ofperforming a thermal oxidization treatment on the semiconductorsubstrate cleaned in the substrate cleaning step under conditionsidentical to the thermal oxidization treatment conditions in thecorrelation acquisition step to form a thermal oxide film on a surfaceof the semiconductor substrate.
 24. The method for forming a thermaloxide film on a semiconductor substrate according to claim 23, whereinthe semiconductor substrate is a silicon wafer and the thermal oxidefilm is a silicon oxide film.
 25. The method for forming a thermal oxidefilm on a semiconductor substrate according to claim 23, wherein theamount of hydrogen atoms is obtained by performing an RBS measurement ofthe chemical oxide film and is calculated from a determined proportionof hydrogen atoms in the chemical oxide film.
 26. The method for forminga thermal oxide film on a semiconductor substrate according to claim 24,wherein the amount of hydrogen atoms is obtained by performing anATR-FT-IR measurement of the chemical oxide film by using a prism formeasuring ATR and is calculated from absorbance of SiH₃ groups around2130 cm⁻¹.
 27. The method for forming a thermal oxide film on asemiconductor substrate according to claim 23, further comprising, afterthe substrate cleaning step and before the thermal oxide film formationstep, a hydrogen-atom-amount measurement step of measuring an amount ofhydrogen atoms contained in a chemical oxide film formed on thesemiconductor substrate by the cleaning performed in the substratecleaning step.
 28. The method for forming a thermal oxide film on asemiconductor substrate according to claim 14, wherein the predeterminedthickness is 1 to 10 nm.
 29. The method for forming a thermal oxide filmon a semiconductor substrate according to claim 15, wherein thepredetermined thickness is 1 to 10 nm.
 30. The method for forming athermal oxide film on a semiconductor substrate according to claim 18,wherein the predetermined thickness is 1 to 10 nm.
 31. The method forforming a thermal oxide film on a semiconductor substrate according toclaim 23, wherein the predetermined thickness is 1 to 10 nm.